Loss of Mass in Breeding Wrens: Stress or Adaptation?

Abstract

Losses of mass in breeding passerines are commonly observed and presumed to reflect physiological stress from activity while feeding nestlings. Analyses of changes of body mass in House Wrens (Troglodytes aedon) show that: (1) females lose ° 13% of their body mass between onset of incubation and fledging, but reattain their original mass for second broods; (2) 50% of mass loss is achieved before hatching is completed; (3) loss of mass is virtually complete before food demands of the nestlings are greatest; (4) change in body mass during the period of highest food demand is independent of brood size; (5) males foraging at rates similar to females show no change in mass. These patterns do not reflect stress. Rather, the timing of mass losses of females may be adaptive by permitting a 23% reduction in power required to remain aloft in flight before the most demanding period of feeding nestlings.